Any help on pyNodes?

Hey I'm trying to understand this example: en.blender.org/index.php/Dev:2.8/Source/Viewport/PyNodeGLSL so what is the basic idea of this? can I just write python code and it compiles in glsl? in runtime? or what am I missing? so f.e. if I would write a blur node in python , what or where should I start? thanks, hope the question is not too confusing...